Many audio reproduction systems are capable of recording, transmitting, and playing back synchronous multi-channel audio, sometimes referred to as “surround sound.” Though entertainment audio began with simplistic monophonic systems, it soon developed two-channel (stereo) and higher channel-count formats (surround sound) in an effort to capture a convincing spatial image and sense of listener immersion. Surround sound is a technique for enhancing reproduction of an audio signal by using more than two audio channels. Content is delivered over multiple discrete audio channels and reproduced using an array of loudspeakers (or speakers). The additional audio channels, or “surround channels,” provide a listener with an immersive listening experience.
Surround sound systems typically have speakers positioned around the listener to give the listener a sense of sound localization and envelopment. Many surround sound systems having only a few channels (such as a 5.1 format) have speakers positioned in specific locations in a 360-degree arc about the listener. These speakers also are arranged such that all of the speakers are in the same plane as each other and the listener's ears. Many higher-channel count surround sound systems (such as 7.1, 11.1, and so forth) also include height or elevation speakers that are positioned above the plane of the listener's ears to give the audio content a sense of height. Often these surround sound configurations include a discrete low-frequency effects (LFE) channel that provides additional low-frequency bass audio to supplement the bass audio in the other main audio channels. Because this LFE channel requires only a portion of the bandwidth of the other audio channels, it is designated as the “.X” channel, where X is any positive integer including zero (such as in 5.1 or 7.1 surround sound).
In traditional channel-based multichannel sound systems, a bass management technique collects the bass from the main audio channels to drive the one or more subwoofers. Because with bass management the main speakers only have to reproduce the higher-frequency portion of the audio signal and not the bass signal, the main speakers can be smaller. Moreover, in traditional channel-based multichannel sound systems the audio signal is output to a specific speaker or speakers in a playback environment.
Audio object-based sound systems use informational data (including positional data in 3D space) associated with each audio object to position the object in the playback environment. Audio object-based systems are indifferent to the number of speakers in the playback environment. And the multitude of possible speaker configurations in playback environments increases the likelihood for bass overload when using traditional bass management systems. In particular, the bass signal is summed by amplitude and as multiple coherent bass signals are added together there is the possibility for playing back bass signals at an undesirably high amplitude. This phenomenon is sometimes called “bass build-up.” In other words, the electrical summation of coherent bass signals tends to overemphasize the result compared to how those signals would sound if each were reproduced acoustically by a full-range speaker. This bass build-up problem is exacerbated when audio object-based audio is used.
“Bass management” (also known as “bass redirection”) is a phrase used to describe the process of collecting the low-frequency signals from a number of audio channels (or speakers) and redirecting it to a subwoofer. Classic bass management techniques use low-pass filters to isolate the low-frequency portion (or bass signal) of audio channel. The bass signal of each audio channel then is summed along with the low-frequency effects signal to form the subwoofer signal that is reproduced using the subwoofer. Speakers typically differ in their ability to reproduce bass. Speakers with smaller woofers (approximately 6″ and less) are less capable of producing very low or deep bass as compared with larger speakers or speakers specifically designed for bass reproduction (such as subwoofers).
Going from mono to stereo to more and more speakers within a sound system, in the end there are all these additional channels, but we still want to distill them down to one signal that we feed the subwoofer. This is because the subwoofer reproduces very low-frequencies and humans don't respond well in terms of directionality to very low frequencies. The perception will be that the subwoofer handles the bass of sounds placed anywhere in the playback environment.
When using audio object-based sound systems the bass build-up problem is exacerbated due mainly to two issues. First, the playback environment may be grouped into playback zones and the bass signal at some zones may not be desirable all the time. Many cinemas have subwoofers in the back walls to represent the bass from the surrounds in the rear speakers and subwoofers from behind the screen for handling the bass from those speakers. For example, the playback environment may be a cinema with the speakers grouped into two playback zones the front of the room (behind the screen) and the rear of the room. Each of the playback zones has a subwoofer. In some cases it may be desirable to reproduce a bass signal on the subwoofer in the rear playback zone but not the front playback zone. The bass frequencies tend to blend better with higher-frequency audio if the bass signal is close to the other sound coming out of the regular speakers that it is associated with.
Another issue is that object audio is unique in that there is size control over the sound. This allows us to spread the sound from one or two speakers to as many as all the speakers. No matter the size is adjusted it is desirable to spread its coverage but not to change the ratio of the bass sound to the main sound.
One simplistic way to overcome these problems is to apply a fixed scaling factor (or gain coefficient) to each of the bass signals. However, this is only correct for the assumed signals, because it is a first order approximation. It is not a precise way of controlling bass buildup.
A more sophisticated bass management technique extracts the bass signal prior to the spatial rendering of any audio objects. The shortcomings of this technique is that it does not support bass management within subset zones of speakers. This means that if there are speakers that should not be included in the bass management the collected bass signal is mixed back into that speaker such that the speaker's bass signal is still being distributed to the subwoofer. Moreover, that speaker is not only reproducing the bass originally destined for it, but bass from all the other bass-managed speakers as well.
Another type of bass management technique uses wave-field synthesis (WFS). This technique scales the gain of each audio object in order to achieve the correct level of bass from a subwoofer. However, it is not possible, in an error-free manner, to transfer a mix of a subwoofer channel between WFS systems having different loudspeaker densities and a different number of loudspeakers. Moreover, there is no intent and no means to directly address bass buildup resulting from the number of loudspeakers involved.